1. Field of the Invention
This invention relates to a structure of a phase shifting mask (PSM) and fabricating the PSM, and more particularly to an alternating PSM in which use of a semiconductor fabricating procedure is combined with ion implantation technology.
2. Description of Related Art
Currently, application of photolithography in the fabricating procedure on a wafer usually includes a PSM, which can increase the resolution of a pattern transferred from the PSM. Generally, there are two common types of PSM; one is a strong PSM and the other one is a weak PSM. The strong PSM further includes a Levenson PSM and an alternating PSM, and the weak PSM further includes a half-tone PSM, a rim PSM and an attenuated PSM.
The alternating PSM is characterized by having a phase shift layer, which appears in alternating positions. The phase shift layer can invert the phase of a light wave when the light travels through it so that light with an inverted phase can interfere with light without an inverted phase. Because of the interference of the light, the contrast of light intensity is increased. Thus, the alternating PSM is commonly used in the conventional technology.
FIG. 1 is a sectional plot schematically illustrating a conventional alternating PSM. Referring to FIG. 1, a transparent substrate 10 including quartz has a number of opaque layers 12 separately covering surface portions of the transparent substrate 10 so that there are a number exposed portions of transparent substrate 10 for transferring the desired pattern. Then, at every other position between the opaque regions 12, a number of phase shift layers 14 cover the transparent substrate 10. The method of forming the phase shift layers 14 usually uses a transparent layer (not shown) formed over the transparent substrate 10 and defines the transparent layer by etching to form the phase shift layers 14.
When the light, which is also a kind of electromagnetic plane wave, is incident on the transparent substrate 10, the light travelling through the transparent part has interfered with the light travelling through the opaque part. FIG. 2A schematically shows the distribution of the electric field (E) versus the position of the transparent substrate determined in FIG. 1 after light has traveled through the alternating phase shifting mask. Referring to FIG. 1 and FIG. 2A, the electric field (E) correlates to the location of the opaque layers 12 and the phase shift layers 14 on the transparent substrate 10. The Emax and Emin in FIG. 2A are the maximum and the minimum of the E distribution corresponding to the transparent part of the transparent substrate 10 and the phase shift layers, respectively. The Emin is a negative quantity.
FIG. 2B schematically shows the distribution of the intensity of light versus the position of the transparent substrate determined in the FIG. 1. Referring to FIGS. 1, 2A and 2B, the intensity (I) of light correlates to the location of the opaque layers 12 and the phase shift layers 14 on the transparent substrate 10. The intensity I also correlates to the electric field with a relation of I.varies.E.sup.2 so that the maximum intensity I, as indicated by Imax, appears at the Emax or Emin.
For a conventional alternating PSM, as is widely used in photolithography and etching, the critical dimension is below 0.25 .mu.m, and is especially suitable for transferring line/space pattern. Because the phase shift layer 14 is formed by etching, some drawbacks are inevitable, for example, defects appear due to poor etching selectivity or bias of transferring line/space pattern.